WASHINGTON (Reuters) - A drug prescribed to prevent
fractures in breast cancer patients whose tumors have spread
may actually help slow the cancer itself, U.S. researchers
reported on Thursday.

They said Zometa, sold by Swiss drug giant Novartis,
appears to have prevented the spread of tumors to the bone.

The study, released in preliminary form ahead of a meeting
of the American Society of Clinical Oncology, suggests the drug
may be useful for more breast cancer patients.

"Tumor cells are continually being released from the
primary tumor," Dr. Rebecca Aft of Barnes Jewish Hospital and
Washington University in St. Louis, said in a statement.

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"It is thought that the bone marrow harbors these cells and
that these cells are likely to evolve into metastatic disease.
We think that zoledronic acid changes the bone marrow so that
cancer cells are unable to lodge there."

Full details of the study will be released June 3 at the
ASCO meeting in Chicago.

Zometa, known generically as zoledronic acid, is in a class
of drugs known as bisphosphonates. They are usually prescribed
to treat or prevent osteoporosis, but Zometa is mainly used to
strengthen the bones of cancer patients whose tumors have
spread to the bone.

Breast and other cancers commonly spread to the bone and
patients can be crippled by the pain and fractures that result.

Aft's team studied 120 women with stage 2 or stage 3 breast
cancer, which has spread into lymph nodes or other areas near
the breast. Some got intravenous infusions of Zometa every
three weeks for a year, while others did not.

At the time of diagnosis, none of the patients had evidence
of any spread when checked using computed tomography (CT) or
positron emission tomography (PET) scans. But bone marrow
samples showed about 40 percent of the patients had detectable
breast tumor cells in their bone marrow.

After treatment, the researchers took more bone marrow
samples.

They said 23 percent of women who got Zometa had tumor
cells in the bone marrow after three months, compared to 36
percent of those who did not get the drug.

"If longer follow-up shows that women without tumor cells
in their bones do not go on to develop metastatic disease, then
it would be reasonable to say that bisphosphonates will likely
benefit women with locally advanced breast cancer," Aft said.